Method for drying material sensitive to overheating



y 7, 1938.- J. M. PEHRSON ET AL 2,117,822

METHOD FOR DRYING MATERIAL SENSITIVE TO OVERHEATING Filed Feb. 6, 1936 MM May 11,1938 2,117,822

UNITED STATES PATENT OFFICE METHOD FOR DRYING MATERIAL SENSI- TIVE T OVERHEATING Johan Marten Pehrson and Ragnar Viktor Pelu'son, Stockholm, Sweden Application February 6, 1936, Serial No. 62,704 In Sweden July 31, 1935 1 Claim. (Cl. 34-24) The present invention relates to a method of unit time. For lower water contents of the mateand means for drying material in a, state of loose rial to be dried it is therefore very difllcult to obaggregation which is sensitive to overheating, tain a satisfactory thermal economy in the pneu- Darticularly such material which in a very moist matic drying, since the water given off by the 5 condition tends to be closely matted or packed drying material does not justify the p r 5 together but is of a more porous or solid state tively a e quant y of drying medium. Fo when possessing a lower content of water. Such high water contents, however, it is apparentthat materials include for example most vegetable considerable e my n be rved in the products, such as grass, clover, income and vines q n y f he t pp 1 0! root crop and other vegetables, but also cer- Nevertheless, the treatment even of material of tain animal products useful a foodstuff or high water content according to the pneumatic fodder. method is very difilcult in practice, ifthe water Earlier methods of drying such products may content is to be reduced to a comparatively low be divided substantially in two groups, namely Thus, the ry n efiect f ry n i5 pneumatic drying and layer-drying, hot gases or medium has so decreased in the latter part of the .5 air being in both cases used as drying medium. p s. wh n the fina st p f y n is to tak According to the first method the material is place, due to the abso 'i f W t i earlier supported and transported by the d ing stages of the process, that particularly sensitive medium, in a comparatively rapid current until or selective devices and a careful manual operathe material has reached the required dryness, 111011 o t e process e required in Order that t e whereupon it is separated from the gases. Acdes d d r e of y ss m y app a y be cording to th o d method th t i is reached. when treating nonhomogeneous matefed into a suitable drying apparatus, being formed rial it is further necessary to combine with the into a comparatively thick layer with which the p o ess a di i r ti operation nv v drying medium is brought into contact. Many tra costs, if an even drying is to be achieved. 25 methods are adapted to work on this principle When treating materials with high water conand of them have proved most efllcient those tent and when it is not required to carry the in which the drying medium is led transversely drying so iar th t the t r app a s the through the charge or layer of material substanhygroscopic S the drying according t0 e tially from below upwards. Particularly good pn matic m h d m y be r d u w v y t results have been obtained if the material has good thermal economy d e t the high p imparted to it simultaneously a mixing motion, lure of h drying m m n because the as is the case for example in a rotating drying danger of overheating is small due to the short drum, period of treatment.

0 methods certain rules for carrying out the dr'yselecting the temperature and the velocity of the ing have, of course, been established with a view drying medium as the latter has no transporting to obtaining the best results. eflfect upon the material to be dried. In particu- Thus the pneumatic drying must work with lar those methods according to this-principle in comparatively high velocities of the drying mewhich the drying medium is led in a current so dium in order that the material may be transtransversely through the material to be dried ported and, if necessary, supported by the dyare characterized by.the readiness with which namic pressure 0! the gases. Since the dimenthe humidity, temperature and velocity of the sions oi the drying apparatus must be kept within drying medium may be adjusted in conformity reasonable limits, the period of treatment will with the state of the material to be dried. At consequently become very short. Consequently, the same time a good thermal economy may be the pneumatic drying is best suited for such obtained even when the material to be dried has materials, that readily give oil their water cona low water content, since the thickness of the tent, this being the case with materials of high layer of material may be so selected that the water content and highly disintegrated material. path of the drying medium through the mate- 50 Due to the fact that in the pneumatic drying rial to be dried always will be sufiiciently long the drying medium also must act as a transportto utilize the drying efiect of the drying medium. irlg medium, the quantity of dryingv material Methods working in the last mentioned mam treated per unit time must be in a certain relaner have thus proved to be superior to all others tion to the quantity of drying medium used per, with regard both to thermal economy and to the In the practical application of these drying In the layer-drying there is more liberty oi 35 possibility of superintending the process so that an even product of desired degree of dryness is obtained. This is particularly the case if a stirring motion is simultaneously given to the material. This will insure that all parts of the layer of material will be equally treated.

These latter methods have, however, the disadvantage that, if the material readily tends to be matted or packed together as is the case with most materials of high water content, a dimculty arises in forcing the drying medium through the layer 01' material. This difliculty is accentuated at high temperatures of the material, as the density of the gases then is so low that a given volume does not displace sufilcient water from the material.

The present invention aims at developing a method of drying material of the aforementioned kind such that the material to be dried at all stages of the process will be treated efiectively. the highest possible thermal economy being observed, the process being at the same time easily superintended and its course easily controlled.

The drying invention according to the present 25 method is characterized therein that the whole drying process is carried out by direct contact between the-material and hot gases in two separate steps in such a way that the material in the first step is introduced into and transported away by a hot current of gas, whereupon, after a substantial separation of the material from said current of gas,'the material is subjected to final drying in a second step by leading hot gases of a lower temperature than the first gas current transversely through a layer of the spread out material.

Thus the drying method according to the invention is a combination of pneumatic drying and layer-drying according to the transverse 40 current principle, so that the advantages of the two methods each known per se may be fully utilized without the combination sufiering from any of the disadvantages of the separate methods. Thus the material may be treated by gases of very high temperature in its most humid state without dimculties and risks and in this way may in a short time be deprived of the main part of its water content while ensuring the highest possible thermal economy. The short period of treatment and the fact that it is not necessary to complete the drying, free water is still left in the material, have the consequence that the danger for overheating the material will be small in spite of the high temperature of the gases. Fur-' thermore by using combustion gases of high temperature and an accordingly small supply of air,

-. the danger of oxidizing thosesubstances in the material sensitive to oxidation will be considerably lessened.

Further, the material may be subjected to final drying to any desired degree of dryness with the greatest possible economy of heat while the drying process is adapted to be fully controlled as the inlet temperature of the drying medium at any moment in the latter part of the process may be adjusted to suit the state of the material, and the quantity of the drying medium may Abe so adjusted as to utilize itwith the greatest efiiciency. Finally even materials which only slowly give off the last .part of their water content may be readily treated without the necessity of subjecting the material to disintegration.

To obtain a uniform product it is of course of the highest importance that the material should be given a stirring motion during the final treatwards towards the lower ment in the second step. This is effected in a simple manner by carrying out the treatment in a rotating drum, in which according to the invention the hot gases are led transversely through the charge.

by a hot gas current,and a drying device of a known kind consisting of a rotating drum, in which hot gases are led transversely through material fed from the first drying device and spread out in a layer, and in addition a device, for example a cyclone separator or the like, for separating the material coming from the first drying device from the accompanying gas current being arranged between the first drying device and a feeding device for the second drying device.

Such a plant for carrying out the mehod is illustrated diagrammatically on the accompanying drawing.

Figs. 1 and2 show two elevations of the plant taken at right angles to each other.

By means of the fan I the material to be dried is brought into the hopper 2 from where it is introduced into the container 4 by means of a transport screw arranged in the channel 3. Hot gases from the furnace 5 enter the said container through the pipe 6.

The gases transport the material to be dried from the container 4 and are driven through the drying channel 1 by the fan 8 and then through the pipe 9 into the cyclone separator II, in which the material and the gases are separated so that the material is discharged through the opening I l and the gases leave at it. The fan I may be arranged either ahead of the cyclone separator, as shown, or after the same. In this drying device the first stage of the drying is carried out.

From the opening H the drying material drops onto-the transport screw it which feeds it into the transverse current drying apparatus It in which the second stage of the drying takes place. This drying apparatus consists of a rotating drying drum in which the material to be dried being spread'out in a layer, is automatically fed longitudinally'towards the discharge end of the drum. The drying drum is provided with openings in its circumference, hot gases of a lower temperature than the gas current used in the first stage, for instance cooled combustion gases from the furnace}, being introduced through the connecting box II to said drum by the fan I. From the connecting box II the combustion gases flow into the drying drum through holes in its circumfereence covered by the material with which the drum has been charged, the gas current flowing transversely through the material which due to the rotation is subjected to a mixing motion.

Those parts ,0! the material to be dried, which at their admission into the container 4 due to their weight cannot immediately be supported current but fall down-.

and transported by the gas part of the container 4, are caught by wings I. of a rotor It (indicated by dashed lines in Fig. 1) carried on the shaft 20. On the rotational the rotor the wings catch the falling material and throw it back upwards into the gas current. This is repeated until the different parts of the material have lost so much in weight due to drying that they are carried away by the gas current. The finally dried material is discharged at the outlet II.

In both of the drying devices the material to be dried is supplied by a transport screw arranged in a feed channel 3 or l3 respectively. The screw and the feed channel are so dimensioned that the material being fed in forms a plug between the walls of the channel in order to prevent as far as possible the entrance of air through the feed channel.

Having now particularly described the nature of our invention and the manner of its operation what we claim is:

A method or drying piece formed material sensitive to overheating, comprising subjecting said material to two separate heating steps, subjecting the material during the first step to hot gases of combustion for a time suflicient only to partially dry said material, utilizing said hot gases for transporting said material to the second heating step, separating the gases used in the first heating step from the partially dried material, and subjecting said partially dried material to further heating by leading hot gases of a .considerably lower temperature than the gases of the first heating step transversely through the material for a period of time suflicient to complete the drying of the material.

JOHAN MARTEN PEHRSON. RAGNAR VIKTOR PEHRSON. 

